Unlikely, but yes. If you're having sex there's always a chance of getting pregnant. No method of "protection" is 100% effective. You had a negative test, which is good, but if you continue to not have a period, and/or if you have signs of pregnancy, go see a doctor, health clinic.

See . See your doctor to rule out an infectious cause. Otherwise you may be dealing with old blood. Common with abnormal periods. A more detailed history and physical are in order.
Please see your doctor.
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Did you miss you. Period? It is appropriate to do the pregnancy test 4-5 days after a missed period. You may wish to see your doctor as soon as one day after the missed period for a more sensitive blood test. Visit this site to check if you have any signs or symptoms of pregnancy.
http://www.webmd.com/baby/guide/pregnancy-am-i-pregnant.
Read more...

Technique. Consult the site below and if you have any symptoms of pregnancy, you may wish to repeat the pregnancy test using first morning urine and follow the instructions carefully.
http://www.webmd.com/baby/guide/pregnancy-am-i-pregnant.
Read more...

See . See your doctor to rule out an infectious cause. Otherwise you may be dealing with old blood. Common with abnormal periods. A more detailed history and physical are in order.
Please see your doctor.
Read more...

Did you miss you. Period? It is appropriate to do the pregnancy test 4-5 days after a missed period. You may wish to see your doctor as soon as one day after the missed period for a more sensitive blood test. Visit this site to check if you have any signs or symptoms of pregnancy.
http://www.webmd.com/baby/guide/pregnancy-am-i-pregnant.
Read more...

Technique. Consult the site below and if you have any symptoms of pregnancy, you may wish to repeat the pregnancy test using first morning urine and follow the instructions carefully.
http://www.webmd.com/baby/guide/pregnancy-am-i-pregnant.
Read more...